Plastic pipe, especially for use in drainage, irrigation, storm sewer and sanitary sewer applications, is produced from high density polyethylene (HDPE). A typical pipe composition contains a high density polyethylene copolymer having a melt flow rate of approximately 0.15 to 0.4 grams per 10 minutes that is blended with carbon black to minimize the effect of ultraviolet light. The Departments of Transportation (DOT) of many states of the United States require plastic pipe used for DOT projects to meet American Association of State Highway Transportation Officials (AASHTO) standards, that include American Society of Testing Materials (ASTM) standards. Current AASHTO standards for corrugated and profile HDPE pipe require the composition of the pipe to have the following properties: a minimum carbon black content of 2 percent by weight; a density of 0.945 to 0.955 grams per cubic centimeter (g/cm3); a melt flow index (MFI) maximum of 0.4; a minimum flexural modulus of 110,000 pounds per square inch (psi); a minimum tensile strength of 3,000 psi; and a minimum stress crack resistance of 24 hours determined by a notched constant tensile load test (NCTL) performed according to ASTM D5397. As used herein, the melt flow index is intended as an equivalent expression to the melt flow rate expressed as grams per 10 minutes at 190° C.
Many commercially available HDPE resins meeting the standards for density, MFI, flexural modulus and tensile strength, fail the NCTL test due to their characteristic broad molecular weight distribution (MWD) that includes the presence of a low molecular weight fraction that contributes to failure of the NCTL test.
To address this problem, specialized narrow MWD, stress crack resistant grades of HDPE have been produced by multistage polymerization to produce a bimodal or multimodal HDPE that when mixed with, for example, about 2 to about 6 percent by weight of carbon black, satisfies AASHTO requirements for corrugated and profile pipe. However, the reactor yield of the specialized HDPE during polymerization typically varies directly with the breadth of the molecular weight distribution. As a result, HDPE resins with narrow MWD are usually sold at a premium.
In another approach, blending of polyethylene resins has been used to address the problem of stress crack resistance. For example, medium density polyethylene pipe blends with improved low temperature brittleness properties and gloss have been obtained, that are composed of HDPE and a concentrate mixture of linear low density polyethylene (LLDPE) and a carbon black, where the LLDPE is a carrier for the carbon black. This approach has the disadvantage that the resulting medium density polyethylene pipe blends have densities (e.g., 0.926 to 0.940 g/cm3) that are too low to meet the AASHTO requirements for corrugated and profile HDPE pipe. Other approaches employ two-stage HDPE polymerization processes to produce bimodal HDPE that is used as a blending component for a resulting medium density polyethylene having a density of 0.930 to 0.940 g/cm3. Similarly, triblends containing a major portion of LLDPE and minor amounts of HDPEs of low molecular weight or high molecular weight have also been reported. However, none of the above methods results in an HDPE having a density of 0.945 to 0.955 g/cm3 and a MFI maximum of 0.4, required by AASHTO for corrugated and profile pipe.